The present invention relates to an electric power steering apparatus.
The electric power steering apparatus assists steering force through motor driving in order to realize an excellent steering feeling.
An electric power steering apparatus disclosed in Japanese Laid-Open Patent Publication No. 2006-131191 has an assist torque calculating unit which calculates assist torque based on a vehicle speed and steering torque. The assist torque calculating unit increases the assist torque as the steering torque becomes larger. The assist torque calculating unit also decreases the assist torque as the vehicle speed V becomes faster. A steering assist force according to this calculated assist torque is applied to the steering wheel through the motor.
An electric power steering apparatus disclosed in Japanese Patent No. 4453012 is provided with a first reference model for setting target steering torque on the basis of a steering angle, and a second reference model for setting a target turning angle of a steering system on the basis of steering torque. Operation of a motor is controlled on the basis of both reference models as ideal models. In such a case, torque feedback control is executed so as to cause actual steering torque to follow target steering torque through the first reference model. As a result, the steering torque can always be set at an optimal value. Moreover, turning angle feedback control is executed so as to cause an actual turning angle to follow a target turning angle through the second reference model. As a result, the actual turning angle according to the steering torque which is input torque can be realized.
Meanwhile, a banked road has a road surface inclined in a width direction thereof and is curved. When a vehicle without the second reference model travels on the banked road, the steered angular position of a steering wheel becomes a steered angular position according to the banked road even if a driver does not apply the steering torque. Thus, the driver does not need to turn the steering wheel greatly while traveling on the banked road.
However, the actual turning angle according to the steering torque is realized in a vehicle with the second reference model, so that the steering wheel is returned to a neutral position unless a driver applies the steering torque while traveling on the banked road. Accordingly, the driver needs to keep applying the steering torque to prevent the steering wheel from returning to the neutral position while traveling on the banked road. Therefore, a suitable steering feeling may not be obtained while traveling on the banked road.